Answers

The Columbia Journalism Review's Who Owns What? database is invaluable for this sort of question. Although I can't get it to spit out a list, you can go through and check manually on each company's ownership of different types of media.

I checked through the Cs and so far came up with CBS, Citadel Broadcasting, Clear Channel, Corus, Cox, and Cumulus as outfits that own more than 25 radio stations.

In addition to that and the above answer, you might want to check out asimilar question which was posted to the Radical Reference site in 2004. You can take a look at the previous list of resources for more sources of information on this subject.